A private network is set up for the exclusive use of a single user, e.g., a company or association. One common form of private network is the private branch exchange (PBX) or the private automated branch exchange (PABX), hereinafter PABX, which is a switching office used by organizations with a large need for internal communications. A PABX is commonly connected to at least one switching office of a public telephone network. One PABX may also be connected by tie trunks, i.e., trunk lines, to other PABXs belonging to the same user to create a PABX network, where each PABX constitutes a switching node of the switching network.
As a result of call routing, call forwarding, call transfer, or conducting a conference call across the PABX network using multiple switching nodes, a call may become connected over more trunks lines than are necessary to complete that call. It will be immediately appreciated that releasing all unnecessary trunk lines will reduce the cost of providing the voice path for the individual call. It will also be noted that reducing the total number of trunk lines required by the PABX network to provide adequate service to all customers will result in lower costs to the network owner.
Typically, PABX networks can be classified as employing centralized control or distributed control. It will be appreciated that PABX networks using centralized network control, e.g., a centralized private network controller such as Signaling System 7, avoid unnecessary trunk line usage through automatic B Channel Conservation features.
Centralized network control systems provide a wide variety of routing and switching of network call traffic with a view towards maximizing network utilization while minimizing call blocking. For example, U.S. Pat. No. 5,142,570 discloses a method for generating a set of routing paths from information including occupancy values. The routing method combines both modified state dependant routing and sequential routing techniques. It should be noted that this patent discusses the relevance of the U.S. Pat. No. 4,788,721. Both of these patents provide details regarding methods for predetermining call routing based on actual network usage data.
On the other hand, U.S. Pat. No. 5,101,451 describes methods for optimizing call routing based in part on the degree of trunk line loading. The '451 patent teaches that availability information for connecting a destination switching system with intermediate switching systems is generated and returned to the source switching system via a common channel whenever a direct circuit is not available between the two systems. In addition, the '451 patent provides a summary of the teachings of the earlier U.S. Pat. No. 4,345,116, which describes yet another method for permitting call path selection on a call-by-call basis.
U.S. Pat. No. 5,058,105 discloses an arrangement for establishing alternate paths in the event of a failure of a communications link where alternate paths are composed of expanding loops around the failed path. Thus, the '105 patent addresses the use of extra capacity, not the maximization of excess capacity. In contrast, U.S. Pat. No. 4,991,204 discloses a communication network which determines primary and alternate calling paths in advance and provides path data to the switching node. The switching node tries the primary calling path and then the alternate calling paths when attempting to complete a call. Finally, U.S. Pat. No. 4,756,019 discloses a system for maximizing revenues on a telephone network by monitoring call usage information using a central computer supplying preferred and alternate routing to the various switching nodes. In such a system, the lowest cost path is always chosen for the call.
It will be appreciated that much effort has been devoted to developing methods and procedures which can be used to route telephone calls between a calling party and the called party. However, little effort has been expended on methods for releasing unnecessary trunk lines, i.e., B Channels, for systems with distributed control systems. Certain applications, e.g., Voice Mail, include automatic release of unnecessary trunk lines. In a large switch application of a private network, trunks may output calls from a front end node that may be routed back, i.e., returned, to the same front end node for such centralized services as an Attendant or a Voice Mail System (VMS). The presence of centralized services depends on automatic B Channel Conservation to avoid congesting inter-machine Primary Rate Interfaces (PRIs).
FIG. 1 shows an example of forwarding to VMS over PRIs in a private network. In FIG. 1, a calling party reaching the network at node 100 is routed to a station on node 200. When the call is not answered, the call is externally forwarded back to VMS system, which is connected to a trunk line off of node 100. Without releasing unnecessary B Channels, i.e., without B Channel Conservation, two B Channels would be allocated during the entire call because of the original call routing. It will be noted that two B Channels are unnecessary because the calling party can be directly connected to VMS within switching node 100. Other than the possibility of a No Answer extension for the PRI Trunk Group incoming to node 200 prior to conversation being initiated, there is no feature being provided by node 200 which is involved in the call. Instead, VMS quickly answers the call which causes B Channel Conservation to automatically release the two unnecessary B Channels. By waiting until conversation is initiated, B Channel Conservation also allows for the possibility of re-routing to a No Answer extension for the PRI Trunk Group incoming to node 200.
The identification and release of unnecessary trunk lines, particularly when more than two switching nodes are connected to complete a call, has not received the same attention as other routing problems.